U.S. Pat. No. 4,863,167 to Matsuki et al. discloses a three piece solid golf ball having improved rebound characteristics and a higher initial velocity which comprises a center portion; an outer layer disposed over said center portion; and a cover disposed over said outer layer. It is preferred for the golf ball to have a higher hardness on the outer layer than the inner layer.
U.S. Pat. No. 5,048,838 to Chikaraishi et al. discloses a three-piece solid golf ball comprising a two-piece solid core consisting of an inner layer and an outer layer and a cover enclosing the core. The Examples illustrate golf balls wherein the inner core layer has the same hardness or is softer than the outer core layer, and wherein the cover composition is softer than both the inner and outer core layers.
U.S. Pat. No. 5,184,828 to Kim et al. discloses a solid three-piece golf ball comprising a core assembly provided by an inner core and an outer layer and a cover characterized by the following features: a) the inner core has a diameter in the range 23-35 mm and hardness (Shore D) in the range 30-62; b) the outer layer has a diameter in the range 36-41 mm and hardness (Shore D) in the range 30-56; c) the golf ball has a maximum hardness (Shore D) in the range of 46-62 at the outer site of the inner core which is located at the interface between the inner core and the outer layer of the golf ball and the hardness then decreases both inwardly and outwardly. The inner and outer core layers are formed from rubber compositions comprising 35-50 parts by weight and 25-40 parts by weight, respectively, of co-cross linking agent, such as zinc diacrylate, and zinc dimethacrylate.
U.S. Pat. No. 7,015,300 to Rajagopalan et al. discloses a golf ball including a core; a layer disposed about the core having a hardness of about 30 Shore D or greater, a flexural modulus of about 1,000 psi to about 80,000 psi, a thickness of about 0.01 inches to about 0.100 inches, and formed from a thermoplastic composition including p-phenylene diisocyanate; and a cover having a hardness of 20 Shore D or greater, a flexural modulus of about 1,000 psi to about 30,000 psi, a thickness of about 0.01 inches to about 0.05 inches; and formed from a thermoset material.
U.S. Pat. No. 6,926,620 to Dalton et al. discloses a golf ball comprising a core of one or more layers, a cover layer of one or more layers and a mantle layer disposed between the core and the cover. The mantle layer comprises a thermoplastic material, preferably having a hardness of greater than 60 Shore D and a Bashore resilience of greater than 34 and a vicat point of greater then 350° F.
Background references also include, for example, U.S. Pat. No. 4,714,253 and 5,002,281 to Nakahara et al. and U.S. Pat. No. 5,935,022 to Sugimoto et al., U.S. Pat. No. 4,781,383 to Kamada et al., U.S. Pat. No. 4,625,964 to Yamada, U.S. Pat. No. 6,838,519 to Rajagopalan et al., U.S. Pat. No. 7,005,479 to Morgan et al., and U.S. Pat. No. 6,905,648 to Sullivan et al.